There is evidence from a variety of animal studies that choroidal blood flow is under neural
control. By contrast, only little information is available from human studies. Recent results
indicate that a light/dark transition is associated with a short lasting reduction in
choroidal blood flow. We have shown that during unilateral dark/light transition both eyes
react with choroidal vasoconstriction strongly indicating a neural mechanism.

The present studies investigate this possibility by using pharmacological interventions. The
pharmacological agents tested include a nitric oxide synthase inhibitor, an alpha-receptor
agonist (as a control substance for the blood pressure increasing nitric oxide synthase
inhibitor), a muscarinic receptor blocker, and a non-specific beta-blocker. These drugs were
chosen on the basis of previous animal experiments, as the systems, which are specifically
influenced by these substances, are likely involved in neural control of choroidal blood
flow.